This paper presents improvements to the plane-based technique for
calibrating freehand 3D ultrasound systems. The improvements are
designed to make it easier for inexperienced users to perform
plane-based calibration and to know that they have got a reliable
result. In particular, we enable the calibration to be performed
using water at room temperature while producing a result that is valid
for average soft tissue, and we show how it is possible to provide
feedback on the reliability of the calibration using a metric based on
the curvature of the calibration criterion function. We present
comprehensive results showing that these innovations improve the
precision of the calibration and offer useful feedback to the user.

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